Object is darker after joining it with another object just like it
It is likely that your normals are flipped. If you used the Mirror command, then it will have flipped them. To confirm if this is the problem, go into edit mode, open the N panel, and turn on Face Normals by checking the box under the Mesh Display tab.

If this is the problem, then you can recalculate or flip your normals by selecting the mesh in edit mode and using the commands under Shading/UVs in the T panel.